This application describes and claims several improvements in the basic electrochemical cell disclosed in U.S. Pat. No. 3,791,871, issued Feb. 12, 1974.
The basic mechanism of operation of the cell described in the aforementioned patent is incorporated by reference in this application. Briefly, the cell utilizes a reactive metal anode highly reactive with water and spaced from a cathode by an electrically insulating film formed on the anode in the presence of water. The anode and cathode are immersed in aqueous electrolyte. In the embodiment shown in the aforementioned patent the anode is formed of an alkali metal such as sodium or lithium and, during operation of the cell, the electrolyte is a liquid solution in water of an alkali metal hydroxide. Alloys and compounds of the alkali metals and other reactive metals should be equally feasible for use as the anode, however, provided they are substantially as reactive with water as are sodium and lithium and further provided, in common with sodium and lithium, they form an insulating film in the presence of water. The electrolyte is preferably an alkali metal hydroxide of the alkali metal utilized as the anode since such hydroxide is naturally formed during operation of the cell and hence automatically regenerates the electrolyte during operation. However, other alkaline electrolytes can be used to initially start up the cell or even during operation of the cell provided they permit the required anode-cathode reactions. Illustratively, potassium and ammonia hydroxide and alkali metal sulfates, halides and others are feasible. After start-up, these electrolytes will become replaced by the hydroxide of the anode metal unless subsequent additions of these electrolytes are made during operation of the cell.
Operation of the cell described in the aformentioned patent involves the following reactions, which for illustrative purposes, utilize lithium as the reactive anode and lithium hydroxide as the electrolyte:
______________________________________ Anode Reaction ##STR1## electrochemical dissolution ##STR2## ##STR3## formation of insultating film on anode ##STR4## direct corrosion/parasitic reaction Cathode Reaction ##STR5## reduction of water ______________________________________
where (aq) represents water and (s) represents a solid salt.
Reaction (1) is necessary for the generation of electricity. Also necessary is an electron consuming reaction similar to reaction (5) but which, ideally, would not result in hydrogen evolution. Reactions (2) and (3) serve to produce the porous insulating film which forms on the anode and protects it. Electrochemical reaction (1) occurs at the base of the flooded pores, the metal-solution interface. Simultaneous with the formation of the film, lithium hydroxide sites at the film-solution interface dissolve into the bulk electrolyte. In order for the electrochemical reaction to proceed at a given constant rate, a steady state situation must exist whereby the electrochemically produced film dissolves into the electrolyte at the same rate as it is formed. Therefore, the electrolyte must have the capacity to dissolve solid salts from the anodic film-electrolyte interface simultaneously with the formation of the salt at the lithium-film interface. If the film dissolves more slowly than it forms, it becomes increasingly thicker and less porous and the electrochemical reaction rate slows down and can approach zero. If the film dissolves more rapidly than it is formed, then a higher reaction rate will result due to the thinner, more porous film. Ultimately, the film could disappear and the lithium become unstable.
Reaction (3) requires a sufficiently high concentration of lithium hydroxide at or near the anode to cause precipitation of f the film as solid lithium hydroxide on the lithium surface. Reaction (4) generates no useful electrical current. Co-pending U.S. patent application Ser. No. 564,984 describes the use of certain organic additives which inhibit the direct corrosion reaction (4).
Reaction (5) actually uses electrical energy and has the disadvantage of hydrogen evolution with its attendant relatively low voltage characteristics. The problem accordingly becomes one of introducing an alternate reaction which will produce the needed exchange of electrons without evolution of hydrogen and which increases efficiency and voltage of the cell.